Predictors and Diagnostic Threshold Values of Functional Dependency in Individuals Aged 80 and Over: A Retrospective Cross-Sectional Study

Aims This study assesses functional dependency rates among Balıkesir residents aged 80 and over, identifying cognitive, nutritional, and depressive predictors of this condition. Method This retrospective cross-sectional study was conducted at the Healthy Aging Center of Balıkesir State Hospital and included 444 individuals aged ≥ 80 years with complete data (January 2024–December 2025). Functional status was assessed using the Katz ADL and dichotomized as independent (6) versus dependent (< 6). Cognitive function (MMSE/MMSE-E), nutritional status (MNA-LF), and depressive symptoms (GDS-15) were evaluated. Total scale scores were used in analyses. BMI and demographic variables were recorded. Results Among 444 oldest-old adults aged ≥ 80 years, 74.55% were women and 70.27% were functionally dependent (Katz ADL < 6). The mean age was 87.21 ± 4.25 years and BMI 26.81 ± 4.31 kg/m²; median MMSE and MNA-LF scores were 25 (22–28) and 26 (24–27), respectively, and mean GDS-15 was 4.23 ± 2.97. Compared with independent participants, dependent individuals were older and had lower cognitive and nutritional scores and higher depressive symptoms ( p < 0.001), while BMI did not differ (p = 0.303); independence was more common in men and in those with ≥ 5 years of education (p < 0.001). In multivariable logistic regression, ≥ 5 years of education was inversely associated with dependence (OR 0.313, 95% CI 0.186–0.528), whereas age, BMI, and GDS-15 increased the odds and MMSE and MNA-LF were protective (Nagelkerke R²=0.409; accuracy 79.3%). ROC analyses yielded AUCs of 0.795 for MMSE, 0.712 for MNA-LF, and 0.695 for GDS-15, with optimal cutoffs of ≤ 26 (73.7% sensitivity, 69.7% specificity), ≤ 25 (50.6% sensitivity, 83.3% specificity), and ≥ 4 (65.7% sensitivity, 72.0% specificity), respectively. Conclusion Functional dependency is highly prevalent among the oldest old and is influenced not only by chronological age but also by cognitive status, nutritional state, depressive symptoms, and body composition. Even mild cognitive impairment (MMSE ≤ 26) was associated with increased dependency. Higher BMI was linked to functional decline, suggesting that the obesity paradox may not apply to functional independence and that sarcopenic obesity mechanisms may contribute. Preventing functional dependence in this age group requires a multidisciplinary approach targeting cognitive reserve, nutrition, depressive symptoms, and BMI monitoring.